Satellite-based positioning systems, such as the Global Positioning System (GPS) and the Global Navigation System Satellite System (GLONASS), have gained a widespread use in many navigation applications. Another example is the planned European system Galileo. These systems utilize a number of orbiting satellites that continuously transmit radio frequency (RF) carrier signals modulated by navigation data and by pseudorandom noise (PRN) digital codes. The navigation data contains satellite ephemeris data describing orbital position of a satellite and other system information. A receiver uses the navigation data to determine the location of the satellite at the time of signal transmission. In addition, by measuring a propagation time, or a transit time, of the satellite signal, the receiver can calculate a range to the satellite.
This satellite-to-receiver range, also known as a pseudorange, is based on measuring the phase offset between the satellite PRN code phase and a replica code generated at the receiver. The phase by which the replica code must be shifted in the receiver to maintain maximum correlation with the satellite code (i.e., approximate propagation time of the signal), multiplied by the speed of light, is approximately equal to the satellite range. For more precise range determination, some receivers derive carrier phase measurements by comparing the phase of the satellite's carrier signal with a phase of an oscillator located within the receiver at an epoch of measurement.
A typical receiver tracks at least four selected satellite signals. With information about the location of each tracked satellite and the pseudorange to each tracked satellite, the receiver can then precisely determine its own three-dimensional position (i.e., latitude, longitude, and altitude), velocity, and local time. For more accurate position determination, two or more receivers may be used together to derive corrections, known as differential corrections. A system using two or more receivers is referred to as a differential system, such as a differential GPS (DGPS) or a differential GLONASS.
Improved accuracy performance allowed the use of satellite-based systems in various aspects of the transportation infrastructure, such as an aviation infrastructure. In commercial aircraft navigation and guidance, satellite-based positioning systems have traditionally been used only for determining position of an aircraft during non-critical portions of a flight, that is, between takeoff and landing. However, in recent years, researchers have started extending these systems for use during landings.
These extended systems have taken the form of ground-augmented systems. Two primary systems that are currently under development are Wide Area Augmentation System (WAAS), which can be used for aircraft en-route navigation and non-precision approaches, and a Local Area Augmentation System (LAAS) that primarily targets precision navigation for landing. The LAAS uses a DGPS, consisting of multiple ground-based reference GPS receivers, a processing station, and a Very High Frequency (VHF) data transmitter.
The ground-based reference GPS receivers, each with a known position, work as normal GPS receivers in determining respective sets of pseudoranges based on signals from at least four satellites. These pseudoranges are fed to the ground-based processing station, which uses the difference between each reference receiver's known position and its satellite-derived position to determine differential corrections. The correction data is then transmitted to an aircraft approaching the landing area via the VHF data link.
The approaching aircraft uses the correction data to correct position estimates of on-board GPS receivers, providing better position solutions than possible using its on-board GPS receivers alone. These corrected position solutions are then compared to a reference-landing path to determine course deviations necessary to ensure the aircraft follows the reference-landing path.
To provide precision approach and landing capabilities, satellite-based navigational systems must adhere to stringent performance requirements such as accuracy, integrity, continuity, and availability of the service. In 1998, Federal Aviation Administration (FAA) initiated a program to establish requirements for developing and deploying a LAAS Ground Facility (LGF). The LGF will monitor the satellite constellation, provide the LAAS corrections and integrity data, and provide approach data to and interface with air traffic control. As a result of this program, the FAA released Specification FAA-E-2937A for a Category I LGF on Apr. 17, 2002, the contents of which are incorporated by reference.
Unfortunately, satellite signals are characterized by low power levels, which make them susceptible to atmospheric disturbances, thermal noise of a receiver, signal blockage, multipath effects, and primarily radio frequency interference (RFI). In fact, the LGF specification has identified the RFI above normal levels as a threat to the LGF that must be handled to ensure accuracy and integrity (i.e., the ability of the system to provide timely warnings to users when the system should not be used for navigation purposes as a result of errors or failures in the system) of the LAAS.
Typically, GPS satellites transmit using two carrier frequencies called L1 (1575.42 MHz), the primary frequency, and L2 (1227.6 MHz), the secondary frequency. Using a spread-spectrum technique called code division multiple access (CDMA), the carrier frequencies are modulated by a unique PRN ranging code sequence assigned to each satellite. In contrast, GLONASS employs a technique called frequency division multiple access (FDMA), in which all satellites transmit the same ranging codes but on different carrier frequencies.
Because satellite-based positioning systems utilize RF carrier signals for transmission of satellite navigation data, the receivers also become susceptible to external RFI signals. The RFI can cause degradation in navigation accuracy or complete loss of receiver tracking. For instance, a GPS receiver can lose lock on a satellite signal due to an interfering RF signal that is only a few orders of magnitude stronger than the minimum received signal strength. To resume tracking, the power of the interferer must be reduced below the tracking threshold of the receiver. Also, during acquisition, when the receiver tries to synchronize its code replica with the received signal, the receiver is susceptible to RFI.
Most of the RFI will come from unintentional, out-of-band sources that can be filtered by a passive filtering in a front-end of the receiver. However, any in-band interference, i.e., the interference that falls within a pass band of a satellite signal, will pass through the front-end of the receiver and may possibly “leak” into the tracking loop pass band, posing a threat to the signal integrity.
Depending on its bandwidth, RFI can be categorized as broadband (wideband), narrowband, or continuous wave (CW). In addition, interference may be pulsed or continuous. Broadband RFI has a flat power spectral density over a wide range of frequencies including GPS L1 band, 1563 through 1587 MHz. Narrowband signals are signals that occupy a small bandwidth (usually less than 500 kHz) centered at a specified carrier frequency. CW signals are also narrowband signals that are modeled a single tone carriers at a specified frequency.
Generally, broadband in-band RFI can be modeled as a white noise with a constant power spectrum density for all frequencies in a satellite signal bandwidth and can be detected by monitoring a signal-to-noise ratio (SNR) at a receiver. Due to the nature of spread-spectrum systems, modulating the satellite with a PRN code results in spreading of the carrier over a large bandwidth. As a result, the received GPS signal combines with the thermal noise of the receiver.
As long as the RFI remains below the thermal noise of the receiver, it will not have significant impact on the performance of the receiver. However, CW and narrowband RFI do not have a constant power spectrum density and are not readily detected by monitoring the SNR. As a result, these signals might “leak” into a tracking pass band of the receiver, severely affecting tracking reliability. It would be beneficial, therefore, to monitor the CW and narrowband RFI present in the tracking loop pass band.